In case where an optical fiber is applied to a submarine cable, the optical fiber is contained in a metal pipe of, for instance, stainless steel, so that the optical fiber is mechanically reinforced, and is sealed to have a water and gas-proof property. The optical fiber thus sealed in the metal pipe is stranded with cores of a power or communication cable to provide a submarine cable. Submarine cables are manufactured by a length of several km. Therefore, a predetermined number of the submarine cables must be connected to each other to provide a predetermined length of a submarine cable crossing a channel, etc. by an outer predetermined number of joints. In this case, an outer diameter of the joints of the connected submarine cable is designed to be equal to that of the remaining portions of the connected submarine cable. In the same manner, metal pipes containing optical fibers are connected to each other to be accommodated in the connected submarine cable, such that an outer diameter of connected portions of the metal pipes is equal to that of the remaining portions of the metal pipes. In this submarine cable, an optical fiber sealed in a metal pipe may be replaced by stranded optical fibers, etc.
Otherwise, an optical fiber is also applied to an underwater probing cable in the form of being contained in a metal pipe in the same manner as explained in the above submarine cable. Even in this use, metal pipes containing optical fibers are connected to each other to have the same diameter at connected portions thereof as that of the remaining portions of the metal pipes. This means that no joint box is used to connect the metal pipe-sealed optical fibers to each other.
In this point, a conventional method for connecting optical fibers sealed in metal pipes will be explained.
At first, each of metal pipes containing optical fibers to be connected is removed at a connecting end by a predetermined length which is, for instance, 50 cm. Then, a sleeve is applied over one of the metal pipes, and the optical fibers are fused to be connected to each other. Then, the sleeve is moved to be positioned over the connected optical fibers, so that the sleeve crosses both removed ends of the metal pipes. In this stage, the sleeve is soldered to the metal pipe ends by means of, for instance, a gas burner. Finally, the soldered sleeve is reduced in outer diameter to be equal to the metal pipes by use of a two-split die. In this diameter reduction of the sleeve, it is elongated by 10 percents, that is, 10 cm in a sleeve of 1 m, so that the connected optical fibers are forced to be pulled in the axial direction in compliance with the elongation of the sleeve, under the state that the optical fibers are not fixed in the metal pipes. Thus, the braking of the connected optical fibers is avoided.
However, the conventional method for connecting optical fibers sealed in metal pipes has a disadvantage in that tensile stress resides in the connected optical fibers, because the surplus length of the optical fibers is approximately 0.05 percents in the metal pipes. In more detail, a length l of the optical fibers which is necessary to be pulled in the axial direction, thereby absorbing the elongation of the sleeve is calculated in the following equation. EQU l.apprxeq.10cm.times.100/0.05.times.1/2=100m
This means that the optical fibers are required to be pulled in the axial direction along the length of 100 m on the both sides of the joint, respectively. Practically, this is impossible to be realized, so that the aforementioned residual tensile stress occurs in the connected optical fibers. As a result, the mechanical strength of the optical fibers is deteriorated, especially, with secular variation under water, and the transmission loss is increased, especially, with secular variation induced by the absorption of hydrogen.
The conventional method for connecting optical fibers sealed in metal pipes has another disadvantage in that a breaking strength of the optical fibers is lowered from 6 kg to 1 kg due to the carbonization of polyurethane layers coating cores of the optical fibers, because the layers are heated at the soldering stage by the gas burner.